Persons suffering from bronchial diseases such as asthma, have a limited lung capacity and the force of the breath is limited. This is even more apparent when the patient is a small child or an infant.
It is therefore of utmost importance that a device which is intended to be used by a small child or an infant is constructed in a manner which reduces the inhalation force and capacity needed to inhale the required amount of substance. It is also important that the tidal volume is small and that the dead space i.e. the space defined between the spacer outlet/valve inlet and the valve outlet/mouth piece/face mask is as small as possible in order to avoid or at least minimize the rebreathing of the exhalation air, e.g. CO.sub.2.
It is therefore important that the valve mounted in the spacer at its outlet opening is constructed in a manner giving the smallest possible resistance and the minimal dead space and that it works in the required manner even when subjected to weak inhalation forces.